Structure, optical absorption and photoluminescence (PL) properties of SiOx films subjected to thermal annealing at 750–1100 °C are investigated. Si crystallites with a few nanometers in size are observed in the SiO1.3 and SiO1.65 films annealed at 1100 °C. Threshold energies in optical absorption of the Si nanocrystallites are higher than that for bulk Si, suggesting a contribution from quantum confinement effects. The PL spectrum shows a remarkable increase in intensity after annealing at temperatures above 1000 °C. This PL behavior is closely related to the formation of Si nanocrystallites by the annealing. The PL peak energy of the annealed films shifts to higher energy with decreasing crystallite size but does not follow the blueshift for the absorption threshold energy. These results suggest that a localized state contributes to the PL mechanism. The SiO1.8 film annealed at 1100 °C, which contains no Si crystallites, exhibits an intense PL similar to the annealed SiO1.3 and SiO1.65 films. It is implied that noncrystalline Si nanoparticles are formed in the SiO1.8 film under high-temperature annealing.

Nuclear-spin-echo decay has been measured in the 'Al frozen core surrounding Cr + in ruby using optical Raman heterodyne detection. Bloembergen's frozen-core model was directly verified by the observation of -1-msec dephasing times in the core compared with 60 @sec in the bulk. Observation of echoes in the ground and optically excited states of "Cr and "Cr allowed separation of direct and indirect Cr + spin-flip contributions to dephasing and hence measurement of the Cr-Cr spin-flip time. The direct dephasing time follows a square-law dependence on concentration, in accord with theory. Indirect dephasing has a square-root dependence on concentration, similar to that observed for optical echoes. Contrary to earlier studies, it is concluded that optical dephasing in ruby, in the concentration range 0.0034 to 0.05 wt % Cr203, is primarily due to magnetic fluctuations produced by Cr + spin flipping; i.e. , that indirect rather than direct dephasing is dominant.

Recent studies by DeVoe and Brewer [Phys. Rev. Lett. 50, 1269 (1983)] have shown that the conventional optical Bloch equations markedly fail to describe the optical saturation behavior of the D, line in the solid Pr'+:LaF&. In this paper we extend these studies to another solid, ruby, using freeinduction-decay observations obtained by pulse excitation of the R, line at 693.4 nm with an ultranarrow-linewidth dye laser. Comparison of the results with Gauss-Markov and randomtelegraph-dephasing theories shows approximate agreement for a fluctuation correlation timẽ , = T2, the dephasing time. This result is remarkably similar to that obtained for Pr':LaF3 However, for theoretical and experimental reasons, we conclude that the theories do not consistently explain the current as well as other data. A qualitative discussion of another dephasing model is given.

We report experimental and theoretical studies of optical hole burning in the inhomogeneous R& line of ruby at low temperatures. Both Stark-shifting and pump-probe techniques using narrow-band single-frequency lasers were employed. In addition to the observation of narrow holes, we have found that a large (up to 70%) decrease in absorption occurs outside the hole and that the relative size of this decrease is constant over the entire inhomogeneous line. This effect is ascribed to fast resonant cross-spin relaxation in the ground-state levels which drives all spins within the optically pumped volume to a common spin temperature. A theoretical model is formulated which describes the power-broadened hole shapes as well as the off-resonance decrease in the absorption coefficient. Fair agreement with experiment is obtained for the case of a magnetic field applied along the~axis; however, sizable deviations from the theory are seen for zero field. We conclude that further studies are needed to elucidate the nature of the Al superhyperfine-broadening mechanism. An upper limit of 1200 nm is deduced for the size of macroscopically broadened regions in ruby.

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